Tuning indicator system



Feb. 3, J D RE";

TUNING INDICATOR SYSTEM Filed Oct. 26, 1940 Hammer W 7 w I M 03 6 0 M Ml. 0 IT 7 2 2W2 a u T f .ence being maximum at resonance" Patented Feb; 3, 1942.

a mmca ronsrsrtml ff; a John D. Reid, "Philadelphia, Pa. ,"assignor to Radip Delaware vCorporation of America, a corporation of Application ctober-2 1940; serial No'. 352, 0 v 10 Claims. J l.; 250 +40) This invention relates to tuning indicator systems for radio signal receiving apparatus and the like, and has for its primary object to provide an improved tuning indicator system forqfrequency's modulation- "signal receiving systems which is'adapted to be connected with a balanced frequencyv modulation detector to indicate accurately the tuning of said system to resonance with a receivedsignah I I I 7 It is a further object of this invention to provide a system for deriving from a frequency mod-- uation balanced detectorcircuit two control potentials which vary in the same sense and at differing rates when tuning through the center frequency of a received signal, the potential differwith said center frequency.- i j It is also a further object of this invention to provide a system for deriving a resultant variable control voltage from the aforesaid potentials, which control voltage has a relatively sharp peak corresponding to resonance or balanced tuning, whereby such voltage may be used for tuning indication and'the like.

The invention will further be understood from the following description when considered. in connection with the accompanying drawing, and

its scope is pointed out in the appended claims.

In the drawing:

Figure 1 is a schematic circuit diagram of a frequency modulation detector provided with a tuning indicator system embodying the invention,

Figures 2, 3 and 4 are graphs. showing curves illustrating certain operating characteristics of the circuit of Fig. 1, and v Figure 5 is a schematic circuit diagram of a portion ,of the circuitofFig. 1 showing a modification of the tuning indicator means;

Referring to Fig. 1, a frequency modulation ide tector comprising apair of diode rectifiers' Band with the center frequency Fo, the output voltage resistor network; as qacross'fthe resistor 9,"

varies in accordance with ithei; curve'El I in. Fig, 2,

v for variations in frequency about resonance. If"

a rectifier llisiprovided, asishofwii; in 'connection with the terminal l5 -and in series with an output impedance or' '-resistor 18 to theterminal IS, a second variable negative voltagev EZd will [be provided, as indicated by the'heavy line curve in the negative half of the eqrvenz, The system thusprovides two negative voltages El and -E2a which vary in the same-sense" and'at differingrates and provide a maximumdifference "at resonance or balanced tuning at the frequency fFoQi The rectifier, I1 is poled'sothat EZa is of like sign withEl in any' case.

The potential El is derivedffr'om' the output a network or thedetect'or across an outputresistor [9 which is connected to the resistor. {through I a filter network comprising; a'se'riesfiresistor 20 'be utilized in a'simil'arm'anner ior deriving two' 'positive voltages which vary. in the same sense '1 are connected in balanced relation with an output resistor network 8 and Band with aninput circuit I0 forming part of a frequency discriminator network I l coupled to the output circuit l2 of a preceding amplifier stage I3, which may be the final stage of an intermediate"frequency amplifier.

As is well known, the output characteristic of a frequency discriminator network and rectifier forming a frequency modulation detector, as in the present example, may be represented as shown in Fig. 2 by the curve E2, this voltage appearing across the output terminals I5 and I6 0f the output resistor network. At resonance and a bypass capacitor Z-l. In thesame manner, I

the rectifier His connected across the entire output ntworkof the detector through afilterv network comprising a series resistor 22 and a bypass capacitor{23,"such filters being provided 'to reduce the audio frequency component appear ingf'across the output circuit-lF-IB. Both resistors l8 and 19 are connected'tofthe low po- QtentiaItermi'naI [6 of the network'which is also fconnected togrOundLas indicated at 24; The Y f utput resistor;'l8 is furtherpr'ovided with a bypass 7 capacitor 25.

ageoutput characteristic E2 f the detector may and at differing rates'with variation-of tuning or frequency about resonance, although the arrangement shown is at present preferred forthe reasonthat the cathodes of therectifiers may be O rated at low-R. F. potential; I I

As vwill'be seen from Fig. 2; the voltages El and EZ'a appearing at terminals 33 and 3| of the output resistors l8 and 19 vary at different rates as the receiving system is tuned through a signal. The difference between E! mam is greatest at the center frequency F0, as indi- I cated at Einax, which is the desire djpoint for cor- "rect tuning.

As a means for utilizing the divergent voltages at resonance for effecting a tuning indication or control voltage having a peak corre It is obvious that the positive half oft he voltsponding to resonance, an amplifier tube 35 is provided in connection with resistors 18 and 19, for differential response to said voltages. For this purpose, the tube is provided With a pair of control grids 36 and 31 located between the cathode 38 and the main anode 39 have positive and negative mutual conductance characteristics with repeat to a third grid or electrode 40 interposed between them so that as a biasing or control potential applied to the grid 36, which may be indicated as gl, is varied in a negative direction, the current to the electrode 40 or g2, when energized positively with respect to the cathode, is decreased, as indicated in Fig. 3 by the curve gl, the current to the electrode 49 being indicated along the ordinate I912.

The mutual conductance of the grid 31 or g3 with respect to the electrode 40 is negative, and the variation in current to the electrode 40 with an increase in the negative biasing potential applied to the grid 31 is indicated by the curve 93 in Fig. 3. The arrangement is such that, with predetermined negative biasing potentials applied to the two grids, equal variations in the values of said potentials in the same sense or direction provides substantially constant current, Ig2, on the electrode 40.

If the voltages applied to the control grids gl and g3, that is, the electrodes 36 and 31, are varied at differing rates but in the same sense ordirection, then the currentto the auxiliary electrode 40 or g3 will reach a maximum or minimum value, depending upon which one of the control grids is higher in voltage, when the difference between the applied voltages is maximum, as at resonance in the present example.

The curve 45 in Fig. 4 shows the effect of applying the voltages El and E211 to the grids 3G and 31, respectively. It will be noted that the current to the auxiliary electrode 43 drops to a relatively sharp peak 46 at the resonance or center frequency F0. By reversing the connections to the grids 36 and 31, the peak may be extended in a positive direction, as indicated at 41. in current to the electrode 49 is provided at resonance.

This change in current is utilized in any suitable manner for controlling an indicating device or other apparatus in connection with the receiving system and, in the present example, is translated into voltage variation through the medium of the output resistor 48 connected in circuit between the electrode 40 and a source of positive anode potential provided by the supply I lead indicated at 49, the ground or negative return lead for which is indicated at 59.

The resistor 48 is suitably bypassed as indicated at and the anode or high potential terminal of the resistor 48 is connected to a control electrode 53 in a tuning indicator tube 54. The latter may be provided with the usual cathode 55, and target electrode 56, which fluoresces when connected to a positive source of current such as the lead 49 over that portion which is not shadowed by the control electrode 53. A tube of this character is known commercially as an RCA 6AF6G. The main anode electrode 39 of the tube 35 is connected also to the positive potential supply source 49.

The variation in anode current to the electrode 40 when tuning through resonance is applied in the form of voltage variation to the control electrode 53 of the tuning indicator tube 54 across the resistor 48. Since this tuning In either case, a relatively sharp change characteristic has a relatively sharp peak, exact resonance is readily indicated. Any other suitable tuning indicating device, however, may be provided in connection with the output circuit from the electrode 40. For example, a modification of the tuning indicating means may be provided as shown in Fig. 5, in which the same reference numerals are applied to like circuits and circuit elements as in Fig. 1.

Referring to Fig. 5, the rectifier I1 is provided to derive a second variable potential responsive to variations in frequency in the same sense as the potential derived from one-half of the output impedance network 8-9 of the discriminator and detector circuit, whereby, at resonance to the center frequency, the potentials have a maximum difference as shown in Fig. 2 and as hereinbefore described.

Likewise, the tube 35 is provided in which the two control electrodes 38 and 31 connected with the output terminals 30 and 3| of the resistors l8 and I9 have positive and negative mutual conductance characteristics with respect to the output electrode 48. The latter is connected through the potential drop producing resistor 48 which is utilized to couple the electrode 40 to the control grid 58 of an amplifier tube 53, whereby a relay device 68 in the output anode circuit BI is actuated in response to the peak 46 or 41 of Fig. 4 as the tuning passes through resonance.

In the present example, operation of the relay 63 causes a pair of contacts 62 to open, thereby permitting signal output from the receiving system. The contacts are connected across the output circuit 63 for the loudspeaker 64 which, in turn, is supplied with signals from the detector through a suitable audio frequency amplifier 65 and a connection 66 including a coupling capacity 61 to the high potential terminal I5 of the output network 8-9.

With this arrangement, exact resonance to a received signal is indicated by operation of the loudspeaker device, while at all other times the receiving system is silent. Any other suitable control circuit may, however, be provided in connection with the output circuit of the tube 59.

I claim as my invention:

1. In a frequency modulation signal receiving system, the combination of a frequency discriminator network, means for applying variable frequency signals thereto, rectifier means connected with said network providing a balanced output circuit, and means for deriving from said output circuit two voltages which vary in the same sense and at differing rates in response to signal frequency variation causing a divergence which maximizes in response to a predetermined signal frequency.

2. In a frequency modulation signal receiving system, the combination of a frequency discriminator network, means for applying variable frequency signals thereto, rectifier means connected with said network providing a balanced output circuit, means for deriving from said output circuit two voltages. which vary in the same sense and at differing rates in response to signal frequency variation causing adivergence which maximizes in response to a predetermined signal frequency, a control tube having control electrodes connected with said last-named means and an output electrode, and means providing a current variation through said output electrode having a relatively sharp frequency.

peak in response to said maximum potential di- I vergence; V 1

'3. In a frequency modulation signal receiving system, the combination offla frequency discriminator network, means for applying variable irequency, signals. thereto, rectifier means connected with said network providing a balanced output circuit, means for deriving from said output'cir? cuit two voltages which vary in the samesense and atdiffering rates in response to signal frequency variation, and means for indicating a maximumdiflference between said voltages as-anv indication of resonance at a predetermined signal 4. In a frequency modulationsignal receiving system, the combination of a frequency discriminator network, rectifier means connected. with said network providing'abalanced output circuit" an output impedance connected acrossonei-half of said balanced output circuit, a second output impedance connected across said output circuit including a s'eriesconnected rectifier device, and means for deriving from said output impedances two signal-responsive variable control potentials which vary in the same sense and at difierent rates'providing a maximum potential difference at a predetermined center frequency. v

5. The combination of a frequency discrimifor differential response tosaid output potentials and having anioutput electrode, an output im pedanc'e in circuit with said outputfielectrodeiand means for utilizing potential :variations therein;

9.;In' a'frequencyfmodulation signal' rec'eiving system, the combination of affrequencyfdiscrimie nator network, .balanced signal rectifier means nator network, means for applying variable ,Ifre-* 5 quency signals thereto, rectifier means connected with said network providing a balanced output circuit, an outputimpedance connectedacross one-half of said balanced output circuit, a second output impedance connected across said output circuit including a series-connected rectifier device, and means for'deriving from said output impedances twovoltages which vary in the same sense and at differing rates with variation in'fre quency of a signal applied to said network, and means for indicating a maximum-difference between said voltages to provide an indication of. resonance at a predetermined mean frequency.

" electrode, and

.6. In a frequency modulationsignal-receiving I system, the combination of a frequency discriminator network, signal rectifier means comprising balanced output circuit for said rectifier devices,

a pair of rectifier devicesconnectedinbalanced relation with said network, means providing a;

means for deriving from said output circuit two,

signal-responsive variable control potentials whichvary in the same sense and at different 1 1 rates providing a maximum potential difference at a predetermined center, frequency, and. means responsive to said maximum-potential difference for indicating resonance to said pedetermined center frequency.

7. In a frequency modulation signal-receiving system, thecombination of a frequency discrimi nator network, rectifier means comprising a pair connected withsaidnetwork, means providing a I e balanced output circuit for said rectifier means, means for. deriving'fro'msaidoutput circuittwo signal-responsive variable control potentials which vary in the same sense and at/difiering' rates providing a maximum potential.,difference""' 'at a predetermined resonance frequency, a1-tubet I having ,twocontrol grids 'ea'ch'connected' to said output I circuit and having an output "electrode nectedlwith saidoutpiitimpedance.

10. The combination of a -frequency .,discriminator network, rectifier means connected with. 7 said network providing a balanced outputcircuit,

,an output impedance, connected across one half' of said balanced output circuitjmeans. providing" a filter in said connection,- asecond 'output im pedance, connectediacross said output circuitincluding a series-connected rectifierjdevice, means I providing afilter in said'last-named connection, 1 means for-deriving.from said output impedances. f; .two signal respon'sive variable control potentials 7 ,which' vary. in the samejsenseiand atdiiferingrf, 1 rates providing a maximum potentialdifference" at a predetermined frequency, zand' meansra l sponsive to said maximum 'potentialdifierence for indicatinga condition of applied to said network. i

' 7 com: DQaEm-f outputcircuit. for said rectifier's, means for ole-1.1

" riving'from said output circuit two signal-respon sive variable control potentials which vary in-the 1 same sense and at different, rates providing a' maximum'potenti'al difference at 'a' predetermined center frequency',-and currc nt-controlling means.v

differentially responsive'to saidpotentialsgforef I -fecting. a current change'having a pe'akfinfr'e-"v spouse-to said maximum potentialgdiffference. 1. j

8. 'j In afrequency modulation signal+receiving fi 'system,;the combination of 'aif'requencyxdiscrimi- 2 nator network, rectifier meanscomprising'a pair of rectifier devices connected in bala'ncedrelation '1 r with said inetwork', means providing, abfalan'cedrf output circuit for said rectifiers means for .de- 1 riving from said output.- circuit .two ysignal responsive variable control potentials which vary{ in the same sense and, at difierent rates providing ,1 a maximum potential difference at a pre'deter mined center frequency, a tube 'having-twofcom 1 'j -tI'01gIidS COIIIIGCtBdTWith said last-named means resonance'to a signal 

